It is well known that for a fixed separation (D) between two rectangular electrodes having a width (W) and a length (L), the output power of a diffusion cooled CO2 slab laser scales linearly with the area of the electrode. The optimum value of the separation D between the electrodes is predominately determined by the gas pressure, the laser's wavelength and the RF frequency of the power supply used to excite the discharge between the two electrodes. Typical gas pressure is about 50-150 Torr; typical wavelength is about 9-11 microns; typical frequencies are about 80-100 MHz. For these ranges of pressures and RF frequencies, the electrode separation D is about 0.05-0.13 inches. At these ranges of pressures, frequencies and electrode separation, the RF voltage applied to the discharge required to optimally operate a diffusion cooled CO2 laser is approximately 225 Volts.
From the well known voltage, current, power relationship I=P/V, the current into the discharge for a constant voltage applied to the electrodes increases linearly with RF power into the discharge, as schematically illustrated by the solid straight line 10 in FIG. 1. From the well known power, voltage, impedance relationship =V2/P, the impedance of the discharge decreases as the inverse of the power as the RF power into the discharge is increased, as schematically illustrated by the dashed curved line 12 in FIG. 1.
Depending upon the laser's parameters, 50 ohms impedance for the discharge of a diffusion cooled CO2 laser's output power typically occurs at about 100-150 W. Assuming 10% laser efficiency, this laser output power occurs for an input RF power into the discharge at about 1000-1500 W. Below this RF power range, the discharge impedance is generally higher then 50 ohms; above this power range, the discharge impedance is generally lower than 50 ohms. Consequently, the challenges in designing the solid state RF power amplifiers, power combiners and the impedance matching networks for effective power coupling into the discharge for the two situations are different.
Typically, solid state RF power combiners are designed to drive 50 ohms or higher loads, as in FM radio transmitters. Solid state RF power combiners for driving low impedance loads (i.e., <<50 ohms) are rather unique to medium and high power diffusion cooled CO2 slab lasers.
The present invention provides RF solid state power combiners for driving low impedance loads (i.e. for driving medium power diffusion cooled slab laser discharges). The techniques disclosed herein also result in reducing the cost of handling the high currents in the power combiner portion of the solid state RF power supply that result from the low impedance levels as the power into the discharge increases. Realizing low impedance outputs in solid state power combiners is important in efficiently utilizing solid state devices in RF power supplies for driving the low impedance of medium to high power diffusion cooled CO2 lasers in order to reduce cost and enhance laser efficiency. As the laser output power increases, a laser power is finally reached where the use of vacuum tube technology is required in order to provide a cost effective laser product. It is believed that the laser power level requiring a transition from solid state to vacuum tube technology for the RF power supply is above 1000 W if the inventive techniques disclosed herein are utilized.
FIG. 2 shows a schematic of the general conventional approach used in solid state RF power supplies for driving diffusion cooled CO2 gas lasers. As an example, the FIG. 2 circuit 20 is configured for a 6 KW output if four 300 W transistors (i.e. Philips BLF 278A) are used in each of the five power amplifiers (PA1-PA5) 22. Constituting each power amplifier 22 is a set of two transistors arranged in a push/pull amplifier configuration. Two of the push/pull amplifiers are arranged in a binary power combiner configuration, as is well known in the state of the art, to form a power amplifier. Conservatively allowing for circuit losses, the approach illustrated in FIG. 2, with the specified transistors and five power amplifiers 22, is sufficient to drive a CO2 laser having approximately 500 W or higher average output power. Driving the power amplifiers 22 in a pulsed mode of operation at duty cycles (Cy) less than unity can yield RF peak power output (Pp) equal to the continuous wavelength (CW) average power (Pa) divided by Cy. In this pulsed case, the impedance of the laser discharge (L) is lower than in the CW case, due to it being more highly driven.
The approach shown in FIG. 2 begins with a transistor oscillator 24 having the desired frequency (e.g., 100 MHz). The output of the oscillator 24 is fed to a preamplifier chain (PrA) 26 such that the output power of the preamplifier PrA is sufficiently large to drive the five power amplifiers (PA1-PA5) 22 into non-linear operation for maximum efficiency, as is well known in the art. The inputs to each of the power amplifiers (PA1-PA5) 22 have equal amplitude and equal phase RF signals to obtain approximately 1200 W of output RF power from each of the five power amplifiers 22, thereby yielding a total output average power output of 6000 W. The output of each of the power amplifiers 22 is fed to an associated directional coupler (DC) 28, the design of which is also well known to those skilled in the art. The signals from the directional couplers (DC1-DC5) 28 are used to measure the forward power (FP) to the discharge as well as the back-scattered power (BP) from the laser discharge.
The forward power signal FP from the directional couplers (DC1-DC5) 28 of the FIG. 2 circuit 20 is used to determine/monitor the output performance of the individual power amplifiers 22 to diagnosis operational problems, if any, such as: are the power amplifiers 22 putting out equal power. The backward scattered power detected by the collection of directional couplers (DC1-DC5) 28 in the FIG. 2 circuit 20 is used to monitor how well the power supply is matched to the laser's discharge (i.e. the load). When the discharge is unlit, the discharge impedance is very high (i.e. little or no current is flowing through the discharge) and a large percentage of the power is reflected back toward the oscillator due to the large impedance mismatch. When the back-scattered power (BP) is large, the large BP signal is used to turn off the power supply to protect it from damage. A pre-igniter is usually used to ignite the discharge (not shown in FIG. 2 for simplicity) and to keep it lit so significant back reflected power does not occur. U.S. Pat. No. 5,434,881 issued on Jul. 18, 1995, discloses an igniter assembly example on a CO2 diffusion cooled slab laser.
There are numerous ways to implement the back reflection protection circuits which are well known to those skilled in the art. Therefore, such a circuit is not shown in FIG. 2. When the discharge is lit, the impedance is low and by design the RF power supply is made to match the lit discharge impedance so that the reflected power is low or zero.
As further shown in FIG. 2, the output of each of the directional couplers (DC1-DC5) 28 is provided to a corresponding impedance matching network (IMN1-IMN5) 30 which is used to match the input impedance of each of the power amplifiers 22 to the impedance of the power combiner (PC) 32. In the prior art, the input impedance of the power combiner 32 is usually made to be 50 ohms. There are numerous well known means to those skilled in the art for accomplishing this impedance match between the power amplifiers 22, the directional couplers 28 and the power combiner 32 by the impedance matching network 28. One common approach is to use a step-up transformer of the type disclosed in U.S. Provisional Application No. 60/810,538, filed on Jun. 2, 2006, by W. S. Robotham Jr., Frederick W. Hauer and Leon A. Newman; Application No. 60/810,538 is hereby incorporated by reference herein.
The power combiner 32 and the final impedance matching network (IMNf) 34 placed before the laser discharge 36 of the FIG. 2 circuit 20 are one of the most difficult items in the design of an efficient RF solid state power supply to drive a diffusion cooled CO2 slab laser having several hundred watts and higher of output power because of the high currents coming together into and leaving the power combiner 32 and the final impedance matching network (IMNf) 34. For commercial applications, the power combiner 32 needs to have as few components as possible for reduced cost. In the prior art for the RF solid state power supplies, the cost of the supply was approximately equal to ½ the cost of the entire laser. The present invention provides techniques for reducing this cost ratio.
To those skilled in the art, there are numerous well known approaches available for designing the power combiner (PC) and the final impedance matching network (IMNf). (See, for example, Microwave Circuit Analysis and Amplifier Design, Chapter 5; Samual V. Liao; Prentice Hall, Inc. pp 161-192). One known approach for the power combiner (PC) is to use the “Wilkinson” power combiner/divider (“An N-Way Hybrid Power Divider” by Ernest J. Wilkinson; IRE Transactions on Microwave Theory and Techniques, January 1960, pp. 311-313: see also pp. 183-184 of the Liao publication). Another known approach is the “binary” power combiner/divider structure as discussed in FIG. 5-4-1 on page 174 of the Liao publication.
The present invention provides two power combiner designs based upon modifications to the above-mentioned Wilkinson and binary PC designs and one completely new broadband preferred power combiner design for realizing an efficient, low cost power combiner to impedance match low impedance loads associated with medium power diffusion cooled CO2 slab lasers over the prior art. A brief discussion of the Wilkinson and the binary power combiners is provided below in order to fully appreciate the merits of the present invention. These two previous approaches are the approaches presently used to impedance match the RF power supply to the discharge impedance of diffusion cooled CO2 lasers that have output powers below 150-200 Watts. These lower power lasers have higher impedances, e.g., around 50 ohms and higher, than possessed by medium and higher power CO2 lasers, as stated above. The present invention reduces the cost of solid state RF power combiners for driving the discharges of diffusion cooled slab lasers having output powers above 150-200 hundred watts and, therefore, having input impedances much lower than 50 ohms. It is important to note that the concepts of the present invention are also applicable to lower power CO2 lasers of either the wave guide or slab laser configuration.
FIG. 3 schematically illustrates a conventional 4-way binary power combiner 40 that is well known to those skilled in the art. The FIG. 3 example has a 50 ohm input from four separate power amplifier sources (PA1-PA4) 42 that have equal amplitudes and phases and delivers an RF output to a 50 ohm load. In this example, the four power amplifiers 42 are shown each having 50 ohm input, and output as is typical in the prior art approaches, using the transistors referenced above. This arrangement provides 5 KW of output power at 100 MHz. A 100 ohm resistor 44 is connected between the outputs of power amplifier PA1 and power amplifier PA2 and between power amplifier PA3 and power amplifier PA4 to dissipate any unbalanced power. These resistors 44 are known as difference resistors or balancing resistors to those skilled in the art. The outputs of power amplifiers PA1/PA2 and power amplifiers PA3/PA4 are combined by the 1:1 ratio transformers T1 and T2 respectfully. The transformers T1 and T2 convert the impedances to 25 Ohms at their respective outputs. A 50 ohm difference resistor 46 is connected between the output of transformer T1 and the output transformer T2, thereby providing a 12.5 ohms output impedance at the 1:1 transformers T3 output. The standard practice has been to match this 12.5 ohm to a 50 ohm load L as shown in FIG. 3. One commonly used approach to accomplish this impedance matching transition is to use a 1:2 step-up transformer T4 to convert the 12.5 Ohm to 50 Ohm to match the load (L) 48. These transformers and resistors are high power devices and, therefore, expensive items.
FIG. 4 illustrates a conventional 4-way Wilkinson combiner 50, again typically having 50 ohm input and output. In this case, four one quarter wavelength co-axial transmission lines 52 are used, each having a 50 ohms characteristic impedance to provide the power combining. Micro strip transmission lines can be and are also often used instead of the co-axial transmission line shown in FIG. 4. The ends of the outer conductor of each of the co-axial transmission lines 52 are attached to electrical ground as shown. One end of each of four resistors 54, each having a value of 50 ohms, is connected to one of the input inner conductors of the four co-axial transmission lines as shown, with the opposite end of each resistor connected to a common electrical connection point. These resistors 54 are the difference or balancing resistors. Each of the four center conductors, at the end of each of the transmission line 52, are connected to another common electrical connection point yielding a 12.50 ohms output impedance. In this prior art example, it is usually desired to connect the common connection point to a 50 Ohms load located some distance away. This is typically accomplished by the use of a λ/4 length, or a multiple ¼ wave length co-axial cable having 25 ohm characteristic impedance, as shown in FIG. 4. The use of ¼ wavelength transmission lines makes this technology narrow band in comparison to the approach of FIG. 3. The balancing resistors are again high cost items.
Variations of the power combiners shown in FIGS. 3 and 4 can be and are used to drive diffusion cooled CO2 lasers having output powers between tens to over 500 watts by adding an impedance matching network (IMNf) in front of the discharge connection and adjusting the IMNf components so that the impedance looking into the IMNf from either of the power combiners of FIG. 3 or 4 is 50 Ohms. The use of such prior art power combiner technologies results in the cost of the RF power supply being approximately equal to the cost of the laser head. The present invention reduces this cost ratio between the RF power supply and the laser head.
For example, elimination of the output impedance transformer T4 in the FIG. 3 power combiner would result in significant cost reduction. In addition, the low output impedance of 12.5 ohms out of the transformer T3 is more closely matched to the low input impedance of the discharge of a medium output power laser. For example, the discharge impedance of a 500 W laser is likely to be about 6 ohms. This closer impedance match also reduces the cost of the IMNf required to match the 12.5 ohms output of the power combiner to the 6 ohms impedance of the discharge. High power RF transformers are expensive and present additional power losses in the power delivery circuits, thereby reducing the wall plug efficiency of the laser.
The high power λ/4 transmission line impedance transformers shown in the FIG. 4 power combiner likewise present additional losses and packaging challenges due to the lengths and diameter of the co-axial line or the need for properly cooling it. Operating at low output impedance for the power combiner results in a lower circuit Q which results in decreased circulating current losses in the impedance matching circuit (IMNf) between the combiner output and the laser discharge.